This invention relates to a signal processing circuit used for an amplifier connected to a rotary magnetic head in a video tape recorder of the helically scanning type.
At a portion containing recording and reproducing amplifier circuits and a rotary magnetic head of a video tape recorder of the helically scanning type, the output terminal of the recording amplifier circuit and the input terminal of the reproducing amplifier circuit are in common connected through a rotary transformer to the rotary magnetic head, and the recording and reproducing amplifier circuits are supplied from the outside with a control voltage for controlling the operation of each circuit. In the recording operation of the video tape recorder, the control voltage is caused to take a prdetermined level so as to control the recording amplifier circuit to be operative and the reproduced amplifier circuit to be inoperative, and the rotary magnetic head is supplied with a recording current from the recording amplifier circuit. On the other hand, in the reproducing operation of the video tape recorder, the control voltage is caused to take another predetermined level so as to control the reproducing amplifier circuit to be operative and the recording amplifier circuit to be inoperative, and signals reproduced by the rotary magnetic head are derived through the reproducing amplifier circuit.
Such a reproducing amplifier circuit as mentioned above is usually constituted fundamentally as shown in FIG. 1. In this previously known circuit, an input terminal 11 is provided at the base of a transistor Q.sub.1 at an amplifying stage, and the emitter of the transistor Q.sub.1 is connected to the parallel connection of a constant current source 12 and a bypass capacitor 13, and a transistor Q.sub.c for supplying with a charging current to the bypass capacitor 13. Further, a control switch S.sub.c which is controlled to be turned on and off by a control voltage V.sub.c is connected to the emitter of the transistor Q.sub.c.
In the recording operation of a video tape recorder employing the circuit shown in FIG. 1, the control switch S.sub.c is maintained to be conductive with a high level taken by the control voltage V.sub.c and consequently the transistor Q.sub.c is made conductive. A current flowing through the transistor Q.sub.c is supplied to the bypass capacitor 13 as the charging current and the bypass capacitor 13 is charged up to have a voltage of a level almost equal to a power supply voltage V.sub.cc across itself. As a result of this, the transistor Q.sub.1 is maintained to be non-conductive so as not to perform amplifying operation. In the reproducing operation of the video tape recorder, the control switch S.sub.c is maintained to be non-conductive with a low level taken by the control voltage V.sub.c and consequently the transistor Q.sub.c is made non-conductive. In this case, the charge stored in the bypass capacitor 13 is discharged through the constant current source 12 so as to reduce the potential at the emitter of the transistor Q.sub.1. As a result of this, the transistor Q.sub.1 is turned on and maintained to be operative to perform amplifying operation.
Accordingly, with the previously proposed recording amplifier circuit as mentioned above, when the control voltage V.sub.C is changed to the high level from low level so that the video tape recorder is changed to be in the reproducing mode from the recording mode, the bypass capacitor 13 is charged up in an instant to have the voltage of the level sufficient to turn the transistor Q.sub.1 off, so that the circuit is changed in a moment to be inoperative to perform an amplifying operation from operative to perform an amplifying operation. On the other hand, when the control voltage V.sub.C is changed to be the low level from the high level so that the video tape recorder is changed to be in the reproducing mode from the recording mode, it takes the bypass capacitor 13 a certain time to discharge to reduce the voltage across itself to be the level sufficient to turn the transistor Q.sub.1 on, so that it takes the circuit the certain time to be changed to be operative to perform an amplifying operation from inoperative to perform an amplifying operation.
In general, the video tape recorder takes the recording mode and the reproducing mode on separate opportunities, respectively, and accordingly the fact that it takes the reproducing amplifier circuit a certain time to commence an amplifying operation after the inoperative state as mentioned above may not cause any particular problem. In a specific case mentioned below, however, this fact is disadvantageous.
A video tape recorder is required to be capable of performing a long time recording operation as one of desirabilities thereof. For complying with this requirement, the running speed of a magnetic tape is reduced so as to make a video track thereon narrow in width. With this treatment, however, the recording characteristics for an audio signal is deteriorated and further tracking control a rotary magnetic head in the reproducing mode is made difficult to be appropriately achieved. Accordingly, it has been proposed to take particular measures generally called a dynamic tracking method to realize the long time recording operation. With the dynamic tracking method, in recording, an audio signal is suppressed in time and recorded in an end portion of each video track separately from a video signal and a pilot signal used for tracking control is frequency-superimposed on the video signal and recorded in each video track, and in reproducing, the rotation path of a rotary magnetic head is controlled with tracking error information obtained from a reproduced pilot signal.
In a video tape recorder employing such particular measures, a magnetic tape is wound on a head drum in an angular area thereon of, for example, larger by 30 degrees than 180 degrees, that is, 210 degrees, and a pair of rotary magnetic heads H.sub.A and H.sub.B which are mounted at angular positions distant by 180 degrees from each other come into contact with the magnetic tape during each scanning period corresponding to one and one sixth (7/6) field periods and having overlapping end portions provided between each two successive scanning periods, so as to scan the magnetic tape alternately every field period. With such scanning, an audio signal S.sub.A suppressed in time is recorded by one of the rotary magnetic heads H.sub.A and H.sub.B during a period corresponding to one sixth (1/6) field periods contained in the beginning portion of each scanning period of 7/6 field periods, and a video signal S.sub.V accompanied with a pilot signal S.sub.P which is frequency-superimposed on the video signal S.sub.V is recorded by the same rotary magnetic head during a successive period corresponding to one field period lF in each scanning period, as shown at X in FIG. 2. Accordingly, as shown in FIG. 3, on the magnetic tape which is scanned by the rotary magnetic heads H.sub.A and H.sub.B in the direction shown with an arrow in FIG. 3, the audio signal S.sub.A suppressed in time is recorded in a portion T.sub.A corresponding to 1/6 field periods of each record track slanted to the direction of tape running at the beginning thereof, and the video signal S.sub.V and the pilot signal S.sub.P are recorded in the remaining portion T.sub.V corresponding to one field of the same record track. In this case, the pilot signal S.sub.P has repeatedly four different frequencies f.sub.1, f.sub.2, f.sub.3 and f.sub.4 in each successive four field periods, respectively. The sudio signal S.sub.A suppressed in time is formed into a pulse code modulation signal and then recorded, for example, in the NRZ modulation.
In the case of such particular measures, as shown at Y in FIG. 2, when the video signal S.sub.V and the pilot signal S.sub.P, which are previously recorded in the portion T.sub.V of each record track marked with P in FIG. 2, are reproduced and the audio signal S.sub.A suppressed in time is recorded in the portion T.sub.A of each record track marked with R in FIG. 2 with monitoring a picture obtained in response to the reproduced video signal S.sub.V, that is, the after recording of the audio signal S.sub.A suppressed in time is performed, recording and reproducing amplifier circuits connected to each of the rotary magnetic heads H.sub.A and H.sub.B are required to be changed so that the recording amplifier circuit is operative, while the reproducing amplifier circuit is inoperative during the period in which the rotary magnetic head H.sub.A or H.sub.B scans the portion T.sub.A of each record track, and the recording amplifier circuit is inoperative, while the reproducing amplifier circuit is operative during the period in which the rotary magnetic head H.sub.A or H.sub.B scans the portion T.sub.V of each record track successive to the portion T.sub.A thereof. In such changes to the recording mode from the reproducing mode or vice versa in respect of the recording and reproducing amplifier circuits, although the change to the recording mode from the reproducing mode is not required to be achieved in a moment but suffices it to be completed within a period in which the rotary magnetic head H.sub.A or H.sub.B is not in contact with the magnetic tape, the change to the reproducing mode from the recording mode is required to be achieved in an instant because the portion T.sub.V is continuous to the portion T.sub.A in each record track. Accordingly, on the occasion of the change to the reproducing mode from the recording mode, it is required that the recording amplifier circuit is changed to be inoperative from operative in a moment and simultaneously the reproducing amplifier circuit is changed to be operative from inoperative in a moment.
However, since the conventional reproducing amplifier circuit shown in FIG. 1 contains the bypass capacitor 13 which is discharged through the constant current source 12, it takes the circuit a certain time to be changed to be operative to perform an amplifier operation from inoperative to perform an amplifying operation, as described above. As a result of this, with such a conventional reproducing amplifier circuit, the video signal S.sub.V and the pilot signal S.sub.P may not be reproduced from the beginning part of each portion T.sub.V on the occasion of the after recording of the audio signal S.sub.A.
In the case where the portion T.sub.A of each recording track is arranged to correspond to more than one field period and the video signal S.sub.V is recorded in both the finishing part of one of the portions T.sub.V and the beginning part of the next of the portions T.sub.V, it is possible to obtain a continuously reproduced video signal even though the video signal S.sub.V is not reproduced from the beginning part of each portion T.sub.V. In such a case, an allowable duration of a period in which the video signal S.sub.V is not reproduced from the beginning part of each portion T.sub.V may be limited practically to be equivalent to less than two horizontal periods. However, in the case of the conventional reproducing amplifier circuit shown in FIG. 1, since the bypass capacitor 13 is required to have the capacitive value of about 0.22 .mu.F in order to allow the reproduced pilot signal S.sub.P having the lowest frequency of about 100 kHz of the aforementioned frequencies f.sub.1, f.sub.2, f.sub.3 and f.sub.4 to pass therethrough and discharged through the constant current source 12, it takes the circuit a time corresponding to more than two horizontal periods to be changed to be operative from inoperative.